Many bipolar transistors have their size set to meet a required collector resistance (Res). Res is proportional to collector resistivity and to the length of the collector between the base and buried layer. Thus, to minimize Res, one typically minimizes both collector resistivity and collector length.
FIG. 1 shows a conventional NPN bipolar transistor 100 consisting of an N-collector 102 formed over an N+ buried layer 104, a P base 106 and an N+ sinker 108 formed in the N-collector 102, an N+ emitter 110 and a P+ base contact 112 formed in the P base 106, and an N+ collector contact 114 formed in the N+ sinker 108. In the conventional bipolar transistor, the collector 102 is doped to the same conductivity throughout. The breakdown voltages BVCEO and BVCBO of the conventional bipolar transistor 100 are both reduced when the resistivity of N− collector 102 is reduced. These breakdown voltages are also reduced when the length of the N− collector 102 is reduced to less than the collector depletion layer thickness at breakdown. Thus, there is a tradeoff between breakdown and Res for bipolar transistors of a given size. Conventional PNP bipolar transistors typically consist of a similar structure but have inverted conductivities.
One approach to increase the breakdown of a transistor with a given collector doping is to cascade the collector with a junction field effect transistor (JFET). The area required for the JFET, however, can consume more area than is saved by reducing the collector doping in some cases so other methods and structures are desired.
Thus, there is a need to overcome these and other problems of the prior art to provide a method and a device to reduce the size of a bipolar transistor while also achieving an improved Res.